1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell. The cell can be of either a primary chemistry, for example a lithium/silver vanadium oxide (Li/SVO) cell, or a secondary chemistry, for example a lithium-ion secondary cell.
Currently, lithium-based primary and secondary cells are used in a large number of medical and commercial applications including implantable medical devices, telephones, camcorders and other portable electronic equipment. They come in a variety of shapes, sizes and configurations as coin, button, and cylindrical and prismatic cells. There are several other applications, however, for which lithium-containing cells may be used but for which present day constructions are unsuitable. Such applications include the next generation of medical instruments, implantable medical devices and surgical tools. For many of these applications, the use of prior art lithium-containing cells is unacceptable because of their shape and construction. In certain types of medical applications, irregularly shaped prismatic cells that are sized for use within the human body are most preferred.
2. Prior Art
Currently, lithium-containing cells are used to power a number of implantable medical devices including ventricular assist devices, artificial hearts and implantable hearing aids, among others. The predominantly used method for manufacturing such cells is to position a single anode and a single cathode overlaying each other with an intermediate separator sandwiched between them. This electrode assembly is then wound together about a mandrel.
A representative wound cell electrode assembly 10 is shown in FIG. 1. The electrode assembly 10 comprises an anode electrode 12 and a cathode electrode 14 disposed on either side of an intermediate separator 16. This anode/separator/cathode structure is then positioned on a plate-shaped mandrel having opposed planar sides (not shown) that is rotated to provide the wound assembly shown. The resulting wound electrode assembly 10 has relatively planar opposed sides 18 and 20 extending to curved ends 22 and 24. The upper and lower edges (only upper edge 26 is shown) of the anode 12, cathode 16 and intermediate separator 14 are also relatively planar.
The electrode assembly 10 is then housed in a prismatic-shaped casing 28 (FIG. 2A) of a deep drawn type. Casing 28 is comprised of opposed major face walls 30 and 32 extending to and meeting with generally planar end walls 34 and 36 at curved corners. The face walls 30, 32 and end walls 34, 36 connect to a planar bottom wall 38. A lid 40 secured to the upper edges of face walls 30, 32 and end walls 34, 36, such as by welding, closes the casing. The lid 40 supports a terminal lead 42 insulated from the lid and casing 28 by a glass-to-metal seal 44. There is also a fill opening 46 in the lid closed by a closure means 48, as is well known by those skilled in the art. The lead 42 is connected to one of the electrodes, typically the cathode, while the casing 28 and lid 40 serve as the lead for the other electrode, typically the anode. This describes a case-negative cell design.
FIG. 2B shows a cylindrically-shaped casing 50 closed by a lid 52 supporting a glass-to-metal seal 54 insulating a terminal lead 56 from the lid. Casing 50 is similar to the casing 28 of FIG. 2A except that it is cylindrical instead of being of a prismatic shape. In this case, the mandrel used to wind the electrode assembly is of a cylindrically shaped rod.
Winding an anode/separator/cathode structure limits the geometric configuration of the resulting cell to cylindrical or generally rectangular shapes. In some applications, these shapes are inefficient because the internal casing volume is grossly under-utilized. For example, the curved ends 20, 22 of electrode assembly 10 fit well into the ends 34, 36 of the prismatic-shaped casing 10 (FIG. 2A) and the upper 26 and lower edges fairly match the shape of the lid 40 and bottom wall 38, respectively. However, if the bottom wall of casing 10 is shaped other than relatively planar, that would not be true. Depending on the shape of the bottom wall 38, there could be a large volume of unused space inside the casing. This is because it is difficult to provide wound electrode assemblies having other than planar upper and lower edges. The use of multiplate electrode assemblies according to the present invention rectifies this problem.
Accordingly, a need exists for electrochemical cells that are, among other things, of a multiplate construction and suitably configured for housing in casings of other than the traditional prismatic shape (FIG. 2A) or cylindrical shape (FIG. 2B). Such “irregularly shaped” electrode assemblies and the casings that house them are particularly well suited for powering implantable medical devices, and the like. Medical devices are being implanted in increasingly disparate parts of the body. For this reason, they must be of varied shapes and sizes, which, in turn, drives the shape of the associated power source. Thus, a process is needed for manufacturing electrochemical cells having shapes that take advantage of as much of the internal volume in a casing, even one of an irregular shape, as possible.